A conventional semiconductor laser device is described below referring to FIG. 18. FIG. 18 is a front view showing the substantial part of a conventional semiconductor laser device. In the figure, the different types of hatching represent facets; they are intended to help identify different layers and do not represent cross sections.
As shown in FIG. 18, a conventional semiconductor laser device 20 has a first cladding layer, an active layer 3, a second cladding layer 4, and an etching stop layer 5 lie over a semiconductor substrate. Over the etching stop layer 5 is a mesa structure 8 having a third cladding layer 6 and a contact layer 7. The first cladding layer, active layer 3, second cladding layer 4, etching stop layer 5, third cladding layer 6 and contact layer 7 constitute a semiconductor crystal-growing portion 30. There is a dielectric film 10, for example, as a silicon oxide film on the etching stop layer 5 and mesa side faces 9. Furthermore, there is an electrode film 12 on the dielectric film 10 and mesa top face 11. This semiconductor laser device 20 has facets which are cleaved perpendicularly to the long side of the laser resonator.
Literature relating to this prior art includes Japanese Patent Laid-open No. 244568/2001 and JP-A No.210915/2001.
The above-mentioned conventional semiconductor laser device 20 has difficulty in providing high power output because there is a considerable catastrophic optical damage (COD) to the resonator's facet for optical output. Therefore, in order to enable high power output of the semiconductor laser device 20, it is necessary to prevent COD to the optical output facet of a resonator having a quantum well active layer. Here, COD refers to a phenomenon that laser light is absorbed on the optical output facet 14 and the generated heat melts and damages the facet and its vicinity.
As a means for preventing COD, a window structure has been suggested in which some impurity is doped near the facet to widen the band gap in the vicinity of the facet. This structure is intended to reduce absorption of laser light with a wider band gap near the facet. In order to realize such a window structure, it is necessary to use an adequate impure element and an adequate method of doping it, which, however, are very difficult to determine. There is also another problem: the impurity doped for the purpose of making a window structure might absorb laser light. Furthermore, in some cases, the window structure does not sufficiently suppress shrinkage of the band gap in the vicinity of the active layer facet.
With this background, as a consequence of many efforts to solve the above problem, the mechanism for deterioration in the COD level in the semiconductor laser device 20 has been found. In that mechanism, the stress of a film above the active layer 3, for example, the dielectric film 10 or electrode film 12, causes a tensile strain on the facet of an active region 17 of the active layer 3, which shrinks the band gap on the facet of the active region 17 and thus encourages absorption of laser light on the facet of the active region. In the semiconductor laser device 20, the band gap for the active layer 3 is determined to decide the output light wavelength depending on the application purpose. Besides, materials for the semiconductor substrate, active layer 3 and cladding layers are determined so as to obtain the required band gap. However, the required band gap is sometimes not obtained because the band gap varies with the stress of the dielectric film 10 and electrode film 12. This occurs more often in facets than inside the semiconductor laser device 20.
Referring to FIG. 18, an explanation is given below in more detail of the mechanism which causes tensile strain on the facet of the active region 17 of the active layer 3 as mentioned above.
The following is a case that the dielectric film 10, made by thermal CVD, and the electrode film 12, made by vacuum evaporation, have tensile strains. Here, stress in the regions of the dielectric film 10 and electrode film 12 which do not cover the mesa structure 8 is referred to as tensile stress A while stress in their regions which cover the surface of the mesa structure 8 is referred to as tensile stress B. Counteraction of tensile stress A compresses the underlying portion of the active layer 3 and brings it into a state of compressive strain. On the other hand, due to the mesa structure 8, the distance between the film portion having tensile stress B and the active region 17 is long, so counteraction of tensile stress B which brings the active region 17 into a state of compressive strain is small. For this reason, tensile strain C is generated by counteraction of the compressive strains of the portions of the active layer 3 which are on both sides of the active region 17. This is a mechanism where by tensile strain C is generated in the active region 17 when films 10 and 12 with tensile stress lie above the active layer 13.
An object of the present invention is to provide a semiconductor laser device which ensures high yield, high reliability and high power output, by decreasing the tensile strain in the active region to improve the COD level.